Triadic synaptic arrangements and their possible significance in the lateral geniculate nucleus of the monkey

Jozsef Hámori, Tauba Pasik, Pedro Pasik, Janos Szentágothai

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Electron microscopic examination of the lateral geniculate nucleus in monkeys reveals the consistent occurrence of triadic synaptic arrangements where a retinal axon terminal is presynaptic to both a principal cell (P-cell) and a non-axonal portion of an interneuron (I-cell) containing flattened vesicles, which in turn is presynaptic to the same P-cell element. These 'triads' are found within glomeruli where the P-cell participates with a dendritic protrusion and the I-cell with a dendritic appendage (type 1 triad), and also outside the glomerulus where the P-cell is represented by its soma or very proximal dendrite (type 2 triad), or the I-cell profile is the soma which again contains synaptic vesicles (type 3 triad). The most parsimonious interpretation of the functional significance of the triadic synapse is to consider it as a device for the inhibitory phasing of the P-cell discharge resulting from the release of I-cell inhibitory transmitter by excitation of the interneuron through the same retinal afferent which causes the initial tonic activity of the P-cell. Further roles of the I-cell are discussed on the basic of a model which considers a random geometry of dendritic (long) and axonal (short) arborizations. The model suggests 3 possible I-cell actions: (1) local inhibition limited to the triadic arrangement; (2) inhibition at a distance through an axonal mechanism; and (3) inhibition at a distance by way of the presynaptic dendrites.

Original languageEnglish
Pages (from-to)379-393
Number of pages15
JournalBrain research
Issue number3
Publication statusPublished - Nov 22 1974

ASJC Scopus subject areas

  • Neuroscience(all)
  • Molecular Biology
  • Clinical Neurology
  • Developmental Biology

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